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OPA380: Composite op-amp theory, mechanism used for precision

Part Number: OPA380

Hi,

I would like to understand a little better the mechanism that the OP380 uses in order to have a low output DC offset while being a current feedback op-amp. In the datasheet it mentions it has another internal op-amp which is auto zeroing connected into the non inverting pin like this:

Could the same strategy be used with any current feedback op-amp? For example if I wanted to create a composite op-amp with the THS3125 and OPA188

Thank you!

  • Hi Andrew,

    I don't think the OPA380 is a current feedback op-amp. It is rather a voltage feedback op-amp that is used for current-to-voltage conversion or trans-impedance application and also performs the DC offset correction.

    I don't think the same strategy can be used with current feedback op-amp, as it becomes unstable if you add a capacitor across its feedback network. So, using the THS3125 and OPA188 to build the trans-impedance circuit similar to the OPA380 would not work. However, you should be able to use a voltage feedback amplifier like the OPA656 or OPA659 along-with the OPA188 to build the trans-impedance circuit similar to the OPA380 with auto-zero DC offset correction.

    Best Regards,

    Rohit

  • In reply to Rohit Bhat:

    Hi Rohit,

    Thanks for the answer. I went through the datasheet of the OPA380 and you are right, there is no mention of current feedback anywhere. I think I assumed it was current feedback since it has high bandwidth and capability of delivering high output current too.

    As you mentioned, the THS3125 would become unstable if there was a capacitor on the feedback loop. Nevertheless, if it was connected with the OPA188 as the OPA380 is internally, there would only be a capacitor across the OPA188 if the 75pF was omitted. Would this not work?

    Thanks again!

    Andrew

  • In reply to Andrew Trico:

    Hi Andrew,

    Yes. There is a possibility that you might be able to make this work without the feedback capacitor across the THS3125. I don't think the 75pF is part of the trans-impedance feedback loop, because it acts as positive feedback for the DC offset correction circuit (OPA188 circuit).

    One thing to note is that I would be careful with the THS3125 trans-impedance gain to not go below the Rf value mentioned in the datasheet for stability purposes.

    Best Regards,

    Rohit

  • In reply to Rohit Bhat:

    Hi Rohit,

    Thanks for the answer. To what bandwidth would you tune the OPA188 circuit? Since it is doing mostly DC offset correction, should I tune it to a low frequency such as 100Hz? Also I will definitely look into the right resistor value for the THS3125, thanks for the note.

    Andrew 

  • In reply to Andrew Trico:

    Hi Andrew,

    Yes. A bandwidth of around 100Hz would be sufficient to tune the OPA188 circuit for DC offset.

    Best Regards,
    Rohit
  • In reply to Rohit Bhat:

    Thank you Rohit